School of Chemistry - Research Publications
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ItemMolecularly isolated perylene diimides enable both strong exciton-photon coupling and high photoluminescence quantum yield(Royal Society of Chemistry, 2019-03-14)Strong coupling in organic media holds the promise of efficient room temperature polariton lasing with solution-processed materials. Currently, however, only five pure-organic materials have been shown to demonstrate polariton lasing. A major challenge is to achieve high exciton–photon coupling while maintaining high photoluminescence quantum yield. Here, we utilize a series of diimide perylene materials that possess sterically hindered substituents, dispersed within a polymer matrix. The rigid structures prevent aggregation and allow high photoluminescence quantum yield (PLQY) at large dye loadings. We demonstrate that these systems can exhibit substantial Rabi splittings at dye loadings that yield film PLQYs of up to 85%, making these perylene derivatives promising materials for polariton lasers.
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ItemHighly Efficient Luminescent Solar Concentrators by Selective Alignment of Donor-Emitter Fluorophores(AMER CHEMICAL SOC, 2019-04-23)Vertically aligning fluorophores to the surface of a waveguide is known to be an effective approach to improve the optical quantum efficiency (OQE) of luminescent solar concentrators (LSCs). While the chromophore alignment assists waveguiding of the emitted photons to the LSC edges, it also significantly reduces the light-harvesting properties of the LSC. We report here a fluorophore pair consisting of a sphere-shaped energy donor and a rod-shaped emitter that was incorporated in LSCs to provide selective fluorophore alignment to address the reduced incident-light absorption issue. A liquid-crystal polymer matrix was used to perpendicularly align the rod-shaped acceptors to a favorable orientation for light guiding, while the sphere-shaped donor was randomly oriented to maintain its light-absorbing properties. The OQE of LSC devices with this selectively aligned donor-acceptor fluorophore system is 78% without significant loss of light-harvesting capability.
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ItemTetrabenzo[5.7]fulvalene: a forgotten aggregation induced-emission luminogen(ROYAL SOC CHEMISTRY, 2019-10-04)Tetrabenzo[5.7]fulvalene, one of the first recognized stable members of mixed fulvalenes, has attracted widespread interest for its remarkable structure. However, little has been known about its photoactivity, most likely owing to its very weak luminescence in the solution state. Here we show for the first time that this compound exhibits aggregation-induced emission (AIE) properties. Its photoluminescence and X-ray crystal structure reveal an interesting mechanism of the AIE phenomenon.
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ItemIntramolecular Versus Intermolecular Triplet Fusion in Multichromophoric Photochemical Upconversion(AMER CHEMICAL SOC, 2019-08-22)Photon upconversion is a process that creates high-energy photons under low photon energy excitation. The effect of molecular geometry on the triplet fusion upconversion process has been investigated in this work through the design and synthesis of four new 9,10-diphenylanthracene (DPA) derivatives by employing platinum octaethylporphyrin as the triplet sensitizer. These new emitter molecules containing multiple DPA subunits linked together via a central benzene core exhibit high fluorescence quantum yields. Interestingly, large differences in the triplet fusion upconversion performance were observed between the derivatives with the meta-substituted dimer showing the closest performance to the DPA reference. The differences are discussed in terms of the statistical probability for obtaining a high-energy singlet excited state from triplet fusion, f, for both inter- and intramolecular processes and the effect of magnetic field on the upconversion efficiency. These results demonstrate the challenges to be overcome in improving triplet fusion upconversion efficiency based on multichromophoric emitter systems.
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ItemNo Preview AvailableImpact of Surface Functionalization on the Quantum Coherence of Nitrogen-Vacancy Centers in Nanodiamonds(AMER CHEMICAL SOC, 2018-04-18)Nanoscale quantum probes such as the nitrogen-vacancy (NV) center in diamonds have demonstrated remarkable sensing capabilities over the past decade as control over fabrication and manipulation of these systems has evolved. The biocompatibility and rich surface chemistry of diamonds has added to the utility of these probes but, as the size of these nanoscale systems is reduced, the surface chemistry of diamond begins to impact the quantum properties of the NV center. In this work, we systematically study the effect of the diamond surface chemistry on the quantum coherence of the NV center in nanodiamonds (NDs) 50 nm in size. Our results show that a borane-reduced diamond surface can on average double the spin relaxation time of individual NV centers in nanodiamonds when compared to thermally oxidized surfaces. Using a combination of infrared and X-ray absorption spectroscopy techniques, we correlate the changes in quantum relaxation rates with the conversion of sp2 carbon to C-O and C-H bonds on the diamond surface. These findings implicate double-bonded carbon species as a dominant source of spin noise for near surface NV centers. The link between the surface chemistry and quantum coherence indicates that through tailored engineering of the surface, the quantum properties and magnetic sensitivity of these nanoscale systems may approach that observed in bulk diamond.
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ItemThe structure and activity of the glutathione reductase from Streptococcus pneumoniae(INT UNION CRYSTALLOGRAPHY, 2019-01)The glutathione reductase (GR) from Streptococcus pneumoniae is a flavoenzyme that catalyzes the reduction of oxidized glutathione (GSSG) to its reduced form (GSH) in the cytoplasm of this bacterium. The maintenance of an intracellular pool of GSH is critical for the detoxification of reactive oxygen and nitrogen species and for intracellular metal tolerance to ions such as zinc. Here, S. pneumoniae GR (SpGR) was overexpressed and purified and its crystal structure determined at 2.56 Å resolution. SpGR shows overall structural similarity to other characterized GRs, with a dimeric structure that includes an antiparallel β-sheet at the dimer interface. This observation, in conjunction with comparisons with the interface structures of other GR enzymes, allows the classification of these enzymes into three classes. Analyses of the kinetic properties of SpGR revealed a significantly higher value for Km(GSSG) (231.2 ± 24.7 µM) in comparison to other characterized GR enzymes.
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ItemElement 25-Manganese(CSIRO Publishing, 2019)This essay in a series being published in the Australian Journal of Chemistry concerns this author’s favourite element, manganese. I was introduced to the chemistry of manganese when undertaking postdoctoral work with George Christou at Indiana University. My first reaction involved the disproportionation of manganese(VII) and manganese(II) to give a trinuclear manganese( III) complex. This one reaction encompasses several of the most appealing aspects of inorganic chemistry – the redox chemistry of metals that exist in multiple oxidation states, the change in a metal ion’s properties upon changing the number of valence electrons, the beautifully symmetric structures of polynuclear metal complexes and, of course, the colours!
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ItemDFT Prediction and Experimental Investigation of Valence Tautomerism in Cobalt-Dioxolene Complexes(AMER CHEMICAL SOC, 2019-04-01)The family of complexes of general formula [Co(Me ntpa)(Xdiox)]+ (tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to successive methylation of the 6-position of the pyridine rings; X = Br4, Cl4, H4, 3,5-Me2, 3,5- tBu2; diox = dioxolene) was investigated by density functional theory (DFT) calculations to predict the likelihood of valence tautomerism (VT). The OPBE functional with relativistic and solvent corrections allowed accurate reproduction of trends in spin-state energetics, affording the prediction of VT in complex [Co(Me3tpa)(Br4diox)]+ (1+). One-electron oxidation of neutral precursor [CoII(Me3tpa)(Br4cat)] (1) enabled isolation of target compounds 1(PF6) and 1(BPh4). Solution variable-temperature UV-vis absorption and Evans method magnetic susceptibility data confirm DFT predictions that 1+ exists in a temperature-dependent valence tautomeric equilibrium between low-spin Co(III)-catecholate and high-spin Co(II)-semiquinonate forms. The solution VT transition temperature of 1+ is solvent-tunable with critical temperatures in the range of 291-359 K for the solvents measured. Solid-state magnetic susceptibility measurements of 1(PF6) and 1(BPh4) reveal the onset of VT transitions above room temperature.
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ItemValence tautomerism and spin crossover in pyridinophane-cobalt-dioxolene complexes: an experimental and computational study(ROYAL SOC CHEMISTRY, 2019-08-21)Compounds [Co(L-N4R2)(dbdiox)](BPh4) (L-N4R2 = N,N'-di-alkyl-2,11-diaza[3.3]-(2,6)pyridinophane, R = iPr (1a), Et (2a); dbdiox = 3,5-di-tert-butyldioxolene) and [M(L-N4iPr2)(dbdiox)](BPh4) (M = Mn (3a), Fe (4a)) have been synthesized and investigated with a view to possible valence tautomeric (VT) or spin crossover (SCO) interconversions. Single crystal X-ray diffraction data for all compounds at 100 or 130 K indicate trivalent metal cations and di-tert-butylcatecholate (dbcat2-) dioxolene ligands. Variable temperature magnetic susceptibility data for all species between 2 and 340 K are consistent with these redox states, with low spin configurations for the cobalt(iii) ions and high spin for the manganese(iii) and iron(iii) ions. Above 340 K, compound 1a exhibits an increase in magnetic susceptibility, suggesting the onset of a VT interconversion from low spin Co(iii)-dbcat to high spin Co(ii)-dbsq (dbsq- = di-tert-butylsemiquinonate) that is incomplete up to 400 K. In solution, variable temperature electronic absorption spectra and Evans NMR method magnetic susceptibility data indicate reversible VT interconversions for 1a in several solvents, with the transition temperature varying with solvent. Variable temperature electronic absorption spectra are temperature-invariant for 3a and 4a, while spectra for 2a in 1,2-dichloroethane suggest the onset of a VT transition at the highest temperatures measured. Density functional theory calculations on all four compounds and literature analogues provide key insights into the relative energies of the different electromeric forms and the possibilities for VT versus SCO interconversions in this family of compounds.
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ItemTetraoxolene-bridged rare-earth complexes: a radical-bridged dinuclear Dy single-molecule magnet(ROYAL SOC CHEMISTRY, 2019-11-07)Two families of neutral tetraoxolene-bridged dinuclear rare earth complexes of general formula [((HBpz3)2RE)2(μ-tetraoxolene)] (RE = Y and Dy; HBpz3- = hydrotris(pyrazolyl)borate; tetraoxolene = fluoranilate (fa2-; 1-RE) or bromanilate (ba2-; 2-RE)) have been synthesised and characterised. In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and each rare earth metal centre has two HBpz3- ligands completing the coordination. Electrochemical studies on the soluble 2-RE family reveal a tetraoxolene-based reversible one-electron reduction. Bulk chemical reduction with cobaltocene affords the cobaltocenium (CoCp+) salt of the 1e-reduced analogue: [CoCp][((HBpz3)2RE)2(μ-ba˙)] (3-RE) that incorporates a radical trianionic form of the bromanilate bridging ligand. Alternating current (ac) magnetic susceptibility studies of 2-Dy reveal slow magnetic relaxation only in the presence of an applied magnetic field, but reduction to radical-bridged 3-Dy affords frequency-dependent peaks in the out-of-phase ac susceptibility in zero applied field. Exchange coupling between the Dy(iii) ions and the radical bridging ligand thus reduces zero-field magnetisation quantum tunnelling and confers single-molecule magnet status on the complex. Comprehensive analysis of the magnetic relaxation data indicates that a combination of Orbach, Raman and direct relaxation processes are required to fit the data for both dysprosium bromanilate complexes.